The present invention relates to the field of turbomachines and to gas turbine engines in particular.
A turbomachine comprises rotors which generally consist of a plurality of elements which are manufactured separately and assembled mechanically. A gas turbine engine thus comprises one or more rotor assemblies each consisting of a compressor and of a turbine. To ensure a certain degree of flexibility as regards the maintenance of the engines, the assemblies are made up of interchangeable modules. In the case of a gas turbine engine rotor, a compressor module is combined with a turbine module. In the context of a repair, for example, it is thus possible to replace the compressor or turbine module of the rotor with another module.
This modular structure of the turbomachines entails a prior balancing of each module. This is because it is desirable not to have to rebalance the complete rotor following the substitution of one module by another.
A method for the modular balancing of a rotor has thus been conceived that allows the standard changing of the components balanced using this method without having to rebalance the rotor.
This method for a gas turbine engine rotor consisting of a compressor module and of a turbine module thus involves separately balancing the modules when they are assembled with equivalent complementary or substitution turbine or compressor modules. The equivalent complementary modules, also termed substitution masses, represent, by virtue of their length, their mass and the position of their center of gravity, the complementary part, in the rotor, of the component to be balanced. Such an assembly is mounted on a balancing machine which makes it possible to determine the unbalance thereof and which provides indications on the correction to be made in the correction planes of the module, that is to say the mass and its position in the plane: radius and angle.
With this method the two modules are balanced as if they were assembled to perfect complementary modules. However, it appears that errors in contacting the modules, the equivalent complementary modules and the tooling distort the balancing. These errors are due to the perpendicularity deviation of the junction face of the module to be balanced with respect to its axis of rotation.
When balancing the module together with the equivalent complementary module, it is possible to discriminate a number of unbalances. As demonstrated by a balancing machine, the total unbalance can be broken down according to the following vector sum:Bt=Bp+Bg+Bo+Ba                 Bt is the total unbalance of the rotor composed of the assembly formed by the module and the equivalent complementary module. Its characteristics are provided by the balancing machine.        Bp is the intrinsic unbalance of the rotor to be balanced resulting from the nonhomogeneity of the materials, from the error in mounting the components which constitute it, and from the distribution of the blades as well. This unbalance is fixed but unknown. It is sought to correct it.        Bg is the geometric unbalance. It is produced by the perpendicularity deviation of the junction face of the module to be balanced with respect to its axis of rotation, which is demonstrated by the substitution module. This unbalance is fixed. It is known from the geometric characterization that is performed systematically for the mounting of the module. It is sought to correct it.        Bo is the unbalance of the tooling, that is to say all the unbalances produced by the errors in the equivalent module, the rotating tooling and also the drive mechanism. These unbalances are cancelled by the operation of inverting the tooling. It will be recalled that this operation involves carrying out a first unbalance measurement at 0° with respect to the reference plane passing through the axis, and a second measurement of the assembly mounted on the tooling by inverting it at 180° with respect to the reference plane.        Ba is the contacting unbalance of the complementary module. It is produced by the eccentricity caused by the error in contacting the complementary module with the module to be balanced. It is variable and unknown. Its amplitude must be determined so as to know what effect it has on the balancing of the rotor as a whole. It is a disruptive unbalance which introduces an error during the modular balancing with a complementary module or substitution mass. More specifically, this error is variable from one mounting operation to another, is not known and cannot be corrected by the inversion operation with respect to the tooling. The nonrepeatability of this type of error can be explained by the variation in the conditions from one mounting operation to another, for example the temperature can vary, and the clamping and positioning can also vary. This unbalance is therefore a disruptive unbalance which it is necessary to keep below a minimum value so that the modular balancing method gives results which can be used. In particular, although the eccentricity resulting from this error is of the same degree as the eccentricity resulting from the geometric error in the junction face of the module, the latter can be minimized or increased without it being possible to anticipate it. Such an error is therefore liable to produce an excess or error value, depending on the circumstances of the mounting operation, of the correcting unbalance applied with the modular balancing method.        
In summary, the contacting error is nonpredictable and nonrepeatable. It disrupts the modular balancing and distorts the results.